1. Field of the Invention
The present invention relates to a load control system having a plurality of control devices and operable to control the amount of power delivered to a plurality of electrical loads from an AC power source, and more particularly, to a novel communication protocol for allowing the control devices of the load control system to communicate with each other.
2. Description of the Related Art
Typical load control systems are operable to control the amount of power delivered to an electrical load, such as a lighting load or a motor load, from an alternating-current (AC) power source. A load control system generally comprises a plurality of control devices coupled to a communication link to allow for communication between the control devices. The control devices of a lighting control system include load control devices operable to control the amount of power delivered to the loads in response to digital messages received across the communication link, or in response to local inputs, such as user actuations of a button. Further, the control devices of a lighting control system often include one or more keypad controllers that transmit commands across the communication link in order to control the loads coupled to the load control devices. An example of a lighting control system is described in greater detail in commonly-assigned U.S. Pat. No. 6,803,728, issued Oct. 12, 2004, entitled SYSTEM FOR CONTROL OF DEVICES, which is incorporated herein by reference in its entirety.
Many prior art lighting control systems use polling techniques to allow the control devices to communicate with each other. In order to execute the polling technique, one control device of the lighting control system must first establish itself as a “master” device. Then, the master device is operable to sequentially transmit poll messages to and receive responses from each of the other control devices in the control system. The response to the poll message may comprise an event to report (e.g., the actuation of a button on a keypad or another high-priority event) or simply a status update message. When a button of a keypad is actuated, the load control devices typically control the lighting loads appropriately. Since the user expects the lighting loads to respond quickly to the actuation of the button of the keypad, the response time of the lighting control system must be rather small, such that the user does not perceive a significant delay.
The master device must complete a polling cycle (i.e., transmitting a poll message to and then receiving a response from each of the control devices in the lighting control system) before repeating the polling cycle. Thus, there can be a rather long time period between when a control device has a high-priority message (corresponding to a high-priority event such as a button actuation) ready to transmit and when the master device polls the control device, thus allowing the control device to transmit the high-priority message. In order to process high-priority messages with an appropriate response time, prior art lighting control systems executing the polling technique either have been limited to a smaller number of control devices in the system (e.g., 32 control devices) or have required a high baud rate (e.g., 125 kbps) to transmit the digital messages between a larger number of control devices (e.g., 64 control devices). When the lighting control systems use a high baud rate, the control devices must be wired together using specific wiring topologies, e.g., a daisy-chain topology, which complicates the installation procedure of the lighting control system. Likewise, limiting the number of control devices that a master device can communicate with to a small number complicates the installation procedure. A lighting control system that is limited to only 32 control devices require the introduction of advanced control devices, such as multi-link processors, to scale beyond the limited number of control devices.
Therefore, there exists a need for a robust communication protocol that uses a polling technique and a low enough baud rate to provide for a free-wiring scheme, while still allowing special events to be executed in a timely manner.